Understanding the Effectiveness of Fluorocarbon Ligands in Dispersing Nanoparticles in Supercritical Carbon Dioxide

To augment the experimental search for a nonfluorous capping ligand that is effective in dispersing nanoparticles in supercritical carbon dioxide, we have developed a simulation protocol, involving atomistic molecular dynamic simulations, which semiquantitatively reproduces empirical observations and hence can be used to gain insight into the relevant physical phenomena. We have used this protocol to examine the reasons behind the exceptional effectiveness of perfluoroalkanethiols, compared to alkanethiols, as capping ligands for nanoparticles in supercritical carbon dioxide. From these simulations, we infer that the principal reason for this enhanced effectiveness is that the C-F bond is much more polar than the C-H bond and hence the fluorocarbons can interact with the quadrupolar carbon dioxide in addition to interacting with its van der Waals centers. For the models studied, the effect of the electrostatic interaction offsets the fact that the dispersion forces exerted by a dense fluorocarbon layer are weaker than in the case of the alkanethiols due to the sparser packing of the former, larger, ligands. The sparse packing and a molecular geometry that ensures that the C-F dipoles of two fluoroalkane molecules on opposing surfaces are always adverse makes two fluorocarbon-passivated surfaces less attractive to each other than the corresponding hydrocarbon-passivated surfaces. Hence, fluoroalkanethiol-passivated surfaces can be both more CO2-philic and less autophilic than alkanethiol-passivated surfaces, which we conclude leads to the effectiveness of fluorocarbon ligands in supercritical carbon dioxide.